feat(slicing): enforce filament flow-type restrictions in auto grouping

Filament grouping already consumed per-filament forbidden nozzle volume
types, but every call site passed an empty map, so a variant-restricted
filament (e.g. one limited to "Direct Drive TPU High Flow") could be
auto-grouped onto an incompatible nozzle flow type on multi-variant
printers.

- add convert_to_nvt_type() to parse extruder variant strings
- add Print::get_filament_unprintable_flow(): forbidden volume types =
  printer extruder variants minus the filament's declared variants;
  filaments declaring no variants stay unrestricted
- feed the map into grouping at the by-object path (Print.cpp) and all
  six mapping/planning sites in reorder_extruders_for_minimum_flush_volume
- unit-test the string parser

Non-restricted configurations produce an empty map, so existing
printers' grouping and g-code are unchanged.

src/libslic3r/PrintConfig.cpp

@@ -689,6 +689,35 @@ std::vector<std::string> save_extruder_ams_count_to_string(const std::vector<std
     return extruder_ams_count_str;
 }
 
+// Maps a full extruder variant string such as "Direct Drive High Flow" to its
+// NozzleVolumeType: the extruder-type prefix is stripped, the remainder is trimmed
+// and looked up as a volume-type name. Returns nvtHybrid when the string cannot
+// be parsed.
+NozzleVolumeType convert_to_nvt_type(const std::string &variant_str) {
+    const auto &ext_types = ConfigOptionEnum<ExtruderType>::get_enum_names();
+
+    auto trim = [](std::string &s) {
+        s.erase(s.find_last_not_of(" \t\r\n") + 1);
+        s.erase(0, s.find_first_not_of(" \t\r\n"));
+    };
+
+    for (auto ext_type : ext_types) {
+        size_t pos = variant_str.find(ext_type);
+        if (pos == std::string::npos)
+            continue;
+
+        std::string result = variant_str;
+        result.erase(pos, ext_type.size());
+        trim(result);
+
+        auto iter = s_keys_map_NozzleVolumeType.find(result);
+        if (iter != s_keys_map_NozzleVolumeType.end())
+            return NozzleVolumeType(iter->second);
+    }
+
+    return nvtHybrid;
+}
+
 // Parses the extruder_nozzle_stats option: one "|"-separated token list per
 // extruder, each token
 // "<volume_type_name>#<count>". Empty string => no per-type stats for that extruder.